Mechanisms of the sparing effect of radiation dose fractionation (sdf) are investigated, using intact insects and insect cells in vitro. Special attention is paid to modification of kinetics of sdf by temperature, because insects are very amenable to temperature manipulation, and to evaluation of repair of noncytokinetic injury, which cannot be scored accurately in mammalian systems. Three manifestations of noncytokinetic injury are under study: Altered longevity in beetles; reduced sexual competitiveness in male beetles; and delayed pupation in diptera. The relative biological effectiveness (RBE) of densely ionizing particles for induction of these phenomena is also under study. Another approach, measuring unscheduled DNA synthesis (UDS), compares mammalian with insect cells, and examines the possibility of induced repair as well as the effect of hyperthermia. BIBLIOGRAPHIC REFERENCES: Glenn, N. D. and H. S. Ducoff, Acute lethality after fast-neutron and X-irradiation of Tribolium confusum. Radiat. Res. 65, 120-129, 1976. Ducoff, H. S., Radiation-induced increase in lifespan of insects : Implications for theories of mammalian aging and radiosensitivity. IAEA Symposium: Biological and environmental effects of low-level radiation, Vol. I, 103-109, 1976.